To improve an output and a torque along a wide rotational speed region from an extremely low opening degree and extremely low rotation to excessive rotation by arranging a fuel blowout hole in a range in which a throttle effect by a lower end surface of a piston valve is generated.

A carburetor is provided with a fuel passage 61 which comprises a fuel taking-out port 63 of a fuel chamber 32, a fuel pipe 65, a fuel leading-in passage 64, an intermediate chamber 66, a fuel feed passage 67, and a nozzle 68. A fuel blowout hole 62 is arranged in the range in which a throttle effect by a lower end surface of a piston valve 41 is generated. It is thus possible to make fuel flow out from a power jet 60 even under an operating condition having a small negative pressure value at the time of from extremely low rotation to low rotation, and from extremely low rotation to low rotation. As a result, a slowdown and stall of an engine at the time of low speed with its throttle wide open is eliminated, and also an output and a torque can be improved along a wide rotational speed range from the extremely low opening degree and extremely low rotation to excessive rotation.